The other way-cool part of Solaris Zones versus UML, BSD Jails, or Xen is that they're tied to resource limits. I hope Xen picks this up - it's great to be able to tell a virtual machine, "If things get busy, you get at most 1/2 a CPU and 512 MB of memory. If no one else is busy, use all you want." We can put 10 dev instances on a machine - each developer thinks they have their own machine (including reboots, root password, etc). The only thing they can't do is load testing - but that's what QA is for, right?